This invention relates to packet and cell switched digital networks and more particularly to a system and method for determining the time interval associated with processing a cell at a network node and for calculating the round trip delay characteristics of traffic between end systems employing a time stamp insertion technique.
Packet switching systems, such as asynchronous transfer mode (ATM), typically rely on switching nodes within a network to transport digital traffic from a source to a destination or from end system to end system. Such systems are frequently operated by service providers who undertake to deliver digital traffic in accordance with a negotiated service agreement. In order to evaluate quality of service (QoS) performance it is frequently desirable to monitor certain service related criteria such as cell transfer delay (CTD) and cell delay variations (CDV). This requires that there be an accurate evaluation of the times involved in transporting data traffic through the network. Typically, delays can occur in the network at the physical layer, at the switching layer or through queuing at switching nodes.
One know solution to measuring delays in digital traffic relates to ATM systems in which Performance Monitoring (PM) cells are used to calculate Cell Delay Variations (CDVs). The known techniques for CDV calculations based on PM cells, however, require that the measuring points (MPs) have a synchronized clock. In any event, the reliability of PM cells to measure delay is still under study.
The ATM Forum Traffic Management Specification version 4.0 includes reference to a loopback cell for use in monitoring specific system parameters. This loopback cell, known herein as an Operations and Maintenance (OAM) cell, is transmitted, on demand, by one of the end systems, e.g. the source, through the various intermediate switching elements in the network to the opposite end system, e.g. the destination, and reports back to the source with relevant information. This information includes on-demand connectivity status, instances of fault detection and in the event of a fault detection, information on the fault location, and pre-service connectivity verification. An OAM cell, in an ATM environment has a fixed length of 53 bytes including a 5 byte header as will be discussed in greater detail later. Advantageously, there are byte location assignments in an OAM cell that have not heretofore been used. In accordance with the present invention these previously unassigned byte locations are used for on-demand round trip delay measurement information.
Accordingly, the present invention provides a method and system for determining the Round Trip Delay (RTD) value which can be used to calculate CTD and CDV. The method and system makes use of the aforementioned loopback OAM cell in the measuring of cell transfer delay in a round trip fashion and to record times required to process cells at network elements. The system and method employ the concept of time stamps. Advantageously, implementation of the invention does not require a synchronization clock.
Therefore in accordance with a first aspect of the present invention there is provided in a packet switched digital network having at least one network element a system for calculating processing intervals at the network element comprising: first means to detect an arrival of a selected packet at the element; first timing means to generate a first time stamp upon detecting the arrival of the packet; second means to note the instance of sending the selected packet from the element; second timing means to generate a second time stamp upon detecting the sending; and means to obtain a processing interval by subtracting the first time stamp from the second time stamp.
In a preferred embodiment of this aspect of the invention the selected cell is a loopback OAM cell which is transmitted, on demand, by an end system through the network and back to the end system.
In this preferred embodiment the network element records the processing interval also known as the delta value in the OAM cell. It is within the scope of the present invention for the OAM cell to record accumulated delta values occurring due to the OAM cell being processed by a number of network elements.
In accordance with a second aspect of the invention there is provided a method of determining a processing interval of a cell at a network element in a packet switched digital network. The method comprises: recording a first time stamp upon arrival of a selected cell at the network element; recording a second time stamp upon sending the selected cell from the element; and obtaining a process interval or delta value by subtracting the value of the first time stamp from the value of the second time stamp.
The present invention also contemplates the calculation of a round trip delay wherein a time stamp is attached to a loopback OAM cell upon being sent from a source, a second time stamp is generated upon arrival at a destination, a third time stamp is generated upon the cell being returned by the destination and a fourth time stamp is generated upon receipt at the source. The delta value, which is the difference between the third time stamp and the second time stamp, is calculated at the destination and recorded in the OAM cell. At the source the round trip delay is determined by subtracting the first time stamp from the fourth time stamp and then subtracting therefrom the delta value.
Therefore, in accordance with a third aspect of the present invention there is provided a method of determining the round trip delay of digital traffic from a source node to a termination node and back to the source through a switched digital network, the method comprising: recording a first time stamp when a selected cell is transmitter by the source node; recording a second time stamp upon receipt of the cell by the termination node; recording a third time stamp when the cell is returned by the termination node; calculating delta by subtracting the second time stamp from the third time stamp; marking the selected cell with the delta value; recording a fourth time stamp upon receipt of the cell by the source node; and determining the round trip delay by subtracting the first time stamp from the fourth time stamp and subtracting delta from the result.
In a preferred embodiment of this aspect the selected cell is an OAM cell. Further, interrupt means are preferably used by the termination node and/or the source node to detect the arrival of the OAM cell.
According to a further aspect of the invention there is provided a system for determining the round trip delay of digital traffic between a source node and a termination node in a switched digital network, the system comprising: first means to record a first time stamp upon transmission of a selected cell by the source node; second means to record a second time stamp upon receipt of the selected cell by the termination node; third means to record a third time stamp and to calculate a delta value by subtracting the second time stamp from the third time stamp; marking means to mark the selected cell with the delta value; fourth means to record a fourth time stamp upon receipt of the selected cell by the source node; and means to calculate the round trip delay by subtracting the first time stamp from the fourth time stamp and subtracting the delta value from the remainder.
In a preferred embodiment of this aspect of the invention the source node and/or termination nodes include interrupt means to ensure prompt detection of the arrival of the selected cell. The selected cell is preferably an OAM cell.